Tiltrotor propulsion system for an aircraft

ABSTRACT

An aircraft includes a fuselage, a forward wing assembly, and aft wing assembly, and a propulsion system. The propulsion system includes a port forward propulsor and a starboard forward propulsor, each of which rotatable between a forward thrust position and a vertical thrust position and together defining a maximum forward thrust capability. The propulsion system also includes a port aft propulsor and a starboard aft propulsor, each of which also rotatable between a forward thrust position and a vertical thrust position, and together defining a maximum aft thrust capability. The maximum forward thrust capability is different than the maximum aft thrust capability to achieve certain efficiencies.

FIELD OF THE INVENTION

The present subject matter relates generally a propulsion system for anaircraft having a plurality of tilt rotors, and an aircraft includingthe same.

BACKGROUND OF THE INVENTION

Aircraft have been developed with a capability for performing verticaltakeoff and landings. Such a capability may allow for the aircraft toreach relatively rugged terrains and remote locations, where it may beimpractical or infeasible to construct a runway large enough to allowfor a traditional aircraft (lacking vertical takeoff capability) totakeoff or land.

Typically these aircraft capable of performing vertical takeoff andlandings have engines that are vectored to generate both vertical thrustand forward thrust. However, an amount of thrust necessary to takeoffand land vertically may not be equal to an amount of thrust required forthe aircraft to maintain forward flight. Accordingly, existing aircraftcapable of performing vertical takeoff and landing include engines thatmay be well suited for generating vertical thrust, but that may not bevery well suited for efficient forward flight. Therefore, an aircraftcapable of performing a vertical takeoff and landing, in addition toachieving more efficient forward flight would be useful.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

In one embodiment of the present disclosure, an aircraft is provided.The aircraft includes a fuselage extending between a forward end and anaft end, a forward wing assembly attached to or formed integrally withthe fuselage proximate the forward end of the fuselage, and an aft wingassembly attached to or formed integrally with the fuselage proximatethe aft end of the fuselage. The aircraft also includes a propulsionsystem including a port forward propulsor and a starboard forwardpropulsor, the port and starboard forward propulsors each attached tothe forward wing assembly on opposing sides of the fuselage androtatable between a forward thrust position and a vertical thrustposition. The port and starboard forward propulsors together define amaximum forward thrust capability. The propulsion system also includes aport aft propulsor and a starboard aft propulsor, the port and starboardaft propulsors each attached to the aft wing assembly on opposing sidesof the fuselage and rotatable between a forward thrust position and avertical thrust position. The port and starboard aft propulsors togetherdefine a maximum aft thrust capability. The maximum forward thrustcapability is different than the maximum aft thrust capability.

In another embodiment of the present disclosure, a propulsion system foran aircraft is provided. The aircraft includes a forward wing assemblyand an aft wing assembly, the forward and aft wing assemblies eachattached to or formed integrally with the fuselage. The propulsionsystem includes a port forward propulsor and a starboard forwardpropulsor, the port and starboard forward propulsors each configured forattachment to the forward wing assembly on opposing sides of thefuselage and rotatable between a forward thrust position and a verticalthrust position. The port and starboard forward propulsors togetherdefine a maximum forward thrust capability. The propulsion system alsoincludes a port aft propulsor and a starboard aft propulsor, the portand starboard aft propulsors each configured for attachment to the aftwing assembly on opposing sides of the fuselage and rotatable between aforward thrust position and a vertical thrust position. The port andstarboard aft propulsors together define a maximum aft thrustcapability, the maximum forward thrust capability being different thanthe maximum aft thrust capability

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 is a perspective view of an aircraft according to variousexemplary embodiments of the present disclosure.

FIG. 2 is a top, schematic of the exemplary aircraft of FIG. 1.

FIG. 3 is a side, schematic view of a side of a wing assembly of theexemplary aircraft of FIG. 1 in a forward thrust position.

FIG. 4 is another side, schematic view of the side of the wing assemblydepicted in FIG. 3 in a vertical thrust position.

FIG. 5 is a side, schematic view of a primary thrust propulsor inaccordance with an exemplary embodiment of the present disclosure.

FIG. 6 is a side, schematic view of a secondary thrust propulsor inaccordance with an exemplary embodiment of the present disclosure.

FIG. 7 is a schematic view of an electric power source in accordancewith an exemplary embodiment of the present disclosure.

FIG. 8 is a top, schematic view of an aircraft in accordance withanother exemplary embodiment of the present disclosure.

FIG. 9 is a top, schematic view of an aircraft in accordance with yetanother exemplary embodiment of the present disclosure.

FIG. 10 is a side, schematic view of a side of a wing assembly of anaircraft in accordance with an exemplary embodiment of the presentdisclosure.

FIG. 11 is a top, schematic view of an aircraft in accordance with stillanother exemplary embodiment of the present disclosure.

FIG. 12 is a top, schematic view of an aircraft in accordance with yetanother exemplary embodiment of the present disclosure.

FIG. 13 is a top, schematic view of an aircraft in accordance with stillanother exemplary embodiment of the present disclosure.

FIG. 14 is a side, schematic view of a supplemental propulsor as may beincorporated in the exemplary aircraft of FIG. 13.

FIG. 15 is a flow diagram of a method for operating a propulsion systemof the gas turbine engine in accordance with an exemplary aspect of thepresent disclosure.

FIG. 16 is a flow diagram of a method for operating a propulsion systemof the gas turbine engine in accordance with another exemplary aspect ofthe present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to present embodiments of theinvention, one or more examples of which are illustrated in theaccompanying drawings. The detailed description uses numerical andletter designations to refer to features in the drawings. Like orsimilar designations in the drawings and description have been used torefer to like or similar parts of the invention. As used herein, theterms “first”, “second”, and “third” may be used interchangeably todistinguish one component from another and are not intended to signifylocation or importance of the individual components. The terms “forward”and “aft” refer to relative positions along an aircraft, with forwardreferring to a position closer to a nose section of the aircraft and aftreferring to a position closer to a tail section of the aircraft. Theterms “port” and “starboard” refer to sides of the aircraft, with portreferring to a side of the aircraft that is to the left when facingforward and starboard referring to a side of the aircraft that is to theright when facing forward.

The present application is directed generally towards an aircraft havingone or more tilt rotors for enabling vertical takeoff and landing. Moreparticularly, an aircraft of the present disclosure includes a fuselage,a forward wing assembly, an aft wing assembly, and a propulsion system.The propulsion system includes a port forward propulsor and a starboardforward propulsor, each of which attached to the forward wing assemblyon opposing sides of the fuselage and rotatable between a forward thrustposition and a vertical thrust position. The port and starboard forwardpropulsors together define a maximum forward thrust capability. Thepropulsion system also includes a port aft propulsor and a starboard aftpropulsor, each of which attached to the aft wing assembly on opposingsides of the fuselage and rotatable between a forward thrust positionand a vertical thrust position. The port and starboard aft propulsorssimilarly define a maximum aft thrust capability. The maximum forwardthrust capability is different than the maximum aft thrust capability,which may lead to certain efficiencies discussed in greater detailherein.

Referring now to the drawings, wherein identical numerals indicate thesame elements throughout the figures, FIG. 1 provides a perspective viewof an exemplary aircraft 10 as may incorporate various embodiments ofthe present invention. FIG. 2 provides a top, schematic view of theexemplary aircraft 10 of FIG. 1. As shown in FIGS. 1 and 2 collectively,the aircraft 10 defines a longitudinal direction L (and a longitudinalcenterline 12 that extends therethrough), a vertical direction V, and atransverse direction T. Additionally, the aircraft 10 defines a portside 14 and an opposite starboard side 16.

The aircraft 10 includes a fuselage 18 extending between a forward end20 and an aft end 22 generally along the longitudinal centerline 12 ofthe aircraft 10, and defining a mean line 24 extending between theforward end 20 and aft end 22 of the fuselage 18 of the aircraft 10. Asused herein, the term “fuselage” generally includes all of the body ofthe aircraft 10, such as an empennage of the aircraft 10. Additionally,as used herein, the “mean line” refers to a midpoint line extendingalong a length of the fuselage 18, not taking into account theappendages of the aircraft 10 (such as the wing assemblies, discussedbelow, or any stabilizers).

The aircraft 10 additionally includes a wing assembly attached to orformed integrally with the fuselage 18. Specifically for the embodimentdepicted, the aircraft 10 includes a forward wing assembly 26 attachedto or formed integrally with fuselage 18 proximate the forward end 20 ofthe fuselage 18 and an aft wing assembly 28 attached to or formedintegrally with the fuselage 18 proximate the aft end 22 of the fuselage18. Notably, for the embodiment depicted, the forward and aft wingassemblies 26, 28 are each configured as two separate wing sections orsides. Specifically, the forward wing assembly 26 includes a port side30 and a starboard side 32 and the aft wing assembly 28 similarlyincludes a port side 34 and an starboard side 36. The port and starboardsides 30, 32 of the forward wing assembly 26 are each separatelyattached to the fuselage 18 approximately at the same location along thelongitudinal centerline 12. Similarly, the port and starboard sides 34,36 of the aft wing assembly 28 are also each separately attached to thefuselage 18 approximately at the same location along the longitudinalcenterline 12. It should be appreciated, however, that in otherembodiments, one or both of the forward wing assembly 26 or aft wingassembly 28 may be formed integrally with the fuselage 18 and/or may beformed of a single, continuous section.

Although not depicted, in other embodiments, the aircraft 10 mayadditionally include one or more stabilizers, such as one or morevertical stabilizers, horizontal stabilizers, etc. Moreover, it will beappreciated, that although not depicted, in certain embodiments, one ormore of the forward wing assembly 26 or aft wing assembly 28 mayadditionally include flaps, such as leading-edge flaps or trailing edgeflaps, for assisting with controlling the aircraft 10 during flight.

Referring still to FIGS. 1 and 2, the exemplary aircraft 10 furtherincludes a hybrid-electric propulsion system 38 for providing theaircraft 10 with a desired amount of thrust during operation. Broadlyspeaking, the exemplary propulsion system 38 includes a port propulsorand a starboard propulsor attached to a wing assembly on opposing sidesof the fuselage 18, an electric power source 40 located remotely fromthe port propulsor and the starboard propulsor, and a primary electriccommunication bus 42 for electrically connecting the electric powersource 40 to the propulsors. Additionally, the primary electriccommunication bus 42 is operable with a controller 45 for distributingelectrical power to the various propulsors through the primary electriccommunication bus 42. Notably, for the embodiment depicted, thepropulsion system 38 additionally includes one or more energy storagedevices 44 (such as one or more batteries) and a secondary electriccommunication bus 46. The one or more energy storage devices 44 areelectrically connected to the primary and secondary electriccommunication buses 40, 46. Additionally, the secondary electriccommunication bus 46 is provided for redundancy purposes, and each ofthe propulsors are additionally electrically connected to the electricpower source 40 through the secondary electric communication bus 46.

Specifically for the embodiment depicted, the propulsion system 38includes a port forward propulsor and a starboard forward propulsorattached to the forward wing assembly 26 on opposing sides of thefuselage 18, as well as a port aft propulsor and a starboard aftpropulsor similarly attached to the aft wing assembly 28 on opposingsides of the fuselage 18. As will be discussed in greater detail below,each of these propulsors are configured as relatively high diameter,principal thrust fans (“PT fans”). Accordingly, the port forwardpropulsor is a port forward PT fan 48, the starboard forward propulsoris a starboard forward PT fan 50, the port aft propulsor is a port aftPT fan 52, and the starboard aft propulsor is a starboard aft PT fan 54.Each of the port forward PT fan 48, starboard forward PT fan 50, portaft PT fan 52, and starboard aft PT fan 54 are in electricalcommunication with the electric power source 40, via the primaryelectric communication bus 42, such that each of the propulsors arepowered by the electric power source 40. It should be appreciated, thatalthough the variety of propulsors are described herein as “fans”, theterm is not intended to limit the present disclosure to any single typeof electric propulsor. Unless specifically limited by the claims, inother embodiments of the present disclosure, any propulsors described asa “fan” herein may additionally, or alternatively, be configured as anyother suitable propulsion device, including, without limitation, ductedfans, un-ducted fans, single stage fans (i.e., fans having a singlestage of propellers), and multiple counter-rotating stage fans (i.e.,fans having a plurality of stages of counter-rotating propellers).

Referring still to FIGS. 1 and 2, the exemplary aircraft 10 depicted isadapted for accomplishing a substantially vertical takeoff and/orlanding, in addition to forward flight. For example, FIG. 1 depicts theaircraft 10 in a vertical takeoff mode and FIG. 2 depicts the aircraft10 and a forward or lateral flight mode.

As will be appreciated, the exemplary aircraft 10 depicted is movablebetween the vertical takeoff mode and horizontal flight mode at least inpart due to each wing assembly including a tilt section. For example,for the exemplary aircraft 10 depicted in FIGS. 1 and 2, the port side30 of the forward wing assembly 26 includes a tilt section 56, thestarboard side 32 of the forward wing assembly 26 includes a tiltsection 58, the port side 34 of the aft wing assembly 28 includes a tiltsection 60, and the starboard side 36 of the aft wing assembly 28includes a tilt section 62. The tilt sections 56, 58, 60, 62 of therespective wing assemblies 26, 28 may be attached to respective staticwing sections (not labeled) of the respective wing assemblies 26, 28 inany suitable manner. For example, the tilt sections 56, 58, 60, 62 maybe attached to respective static wing sections using a swivelconnection, a slip ring interface, or in any other suitable manner.Additionally, for the embodiment depicted, each of the PT fans 48, 50,52, 54 are attached to a respective the tilt section 56, 58, 60, 62 ofthe respective wing assemblies 26, 28. Specifically, for the embodimentdepicted, the port forward PT fan 48 is attached to the tilt section 56of the port side 30 of the forward wing assembly 26, the starboardforward PT fan 50 is attached to the tilt section 58 of the starboardside 32 of the forward wing assembly 26, the port aft PT fan 52 isattached to the tilt section 60 of the port side 34 of the aft wingassembly 28, and the starboard aft PT fan 54 is attached to the tiltsection 52 of the starboard side 36 of the aft wing assembly 28.

Moreover, referring briefly also to FIGS. 3 and 4, side, schematic viewsof a side of a wing assembly of an aircraft 10 is provided in twooperating modes. For example, in certain embodiments, the wing depictedmay be a port side 30 of the forward wing assembly 26, described abovewith reference to FIGS. 1 and 2. As is depicted, each of these tiltsections 56, 58, 60, 62 is movable between a horizontal/forward flightposition (FIGS. 2 and 3) and a vertical flight position (FIGS. 1 and 4).Movement of the tilt sections 56, 58, 60, 62 between the horizontalflight position and the vertical flight position additionally moves therespective PT fans 48, 50, 52, 54 between a forward thrust position anda vertical thrust position. Accordingly, each of the port and starboardforward PT fans 48, 50 and port and starboard aft PT fans 52, 54 aremovable between a forward thrust position and a vertical thrust positionby the respective tilt sections 56, 58, 60, 62. Specifically, each ofthe tilt sections 56, 58, 60, 62 rotates at least about 90° between thehorizontal flight position in the vertical flight position to rotate therespective PT fans 48, 50, 52, 54 between the respective forward thrustpositions and vertical thrust positions. It should be appreciated,however, that in other exemplary embodiments, the plurality of PT fans48, 50, 52, 54 may alternatively be moveable between the forward thrustpositions and the vertical thrust positions in any other suitablemanner. For example, in other embodiments, one or more of the PT fans48, 50, 52, 54 may include a hinge assembly for tilting the PT fan atleast about ninety degrees between the forward thrust position and thevertical thrust position.

Referring still to FIGS. 1 and 2, the exemplary propulsion system 38depicted further includes a plurality of relatively low diameter,secondary thrust fans (“ST fans”) in addition to the plurality of PTfans 48, 50, 52, 54. For example, the exemplary propulsion system 38depicted includes an ST fan on each wing side of each wing assembly.Specifically, the propulsion system 38 includes a port forward ST fan64, a starboard forward ST fan 66, a port aft ST fan 68, and a starboardaft ST fan 70. For the embodiment depicted, each of the ST fans 64, 66,68, 70 are also attached to a respective tilt section 56, 58, 60, 62 ofthe respective wing assemblies 26, 28. Accordingly, the port forward STfan 64 is attached to the same tilt section 56 as the port forward PTfan 48, the starboard forward ST fan 66 is attached to the same tiltsection 58 as the starboard forward PT fan 50, the port aft ST fan 68 isattached to the same tilt section 60 as port aft PT fan 52, and thestarboard aft ST fan 70 is attached to the same tilt section 62 as thestarboard aft PT fan 54. Accordingly, each of the ST fans 64, 66, 68, 70are also movable between a forward thrust position and a vertical thrustposition, for the embodiment depicted, by rotation of the respectivetilt sections 56, 58, 60, 62 of the respective wing assemblies 26, 28.Again, however, it should be appreciated that in other exemplaryembodiments one or more of the plurality of ST fans 64, 66, 68, 70 mayalternatively be moveable between the forward thrust positions and thevertical thrust positions in any other suitable manner. For example, inother embodiments, one or more of the ST fans 64, 66, 68, 70 may includea hinge assembly for tilting the ST fan at least about ninety degreesbetween the forward thrust position and the vertical thrust position.

Moreover, for the embodiment depicted, each of the ST fans 64, 66, 68,70 are spaced from the respective PT fans 48, 50, 52, 54 along thetransverse direction T of the aircraft 10. Specifically, for theembodiment depicted, each of the ST fans 64, 66, 68, 70 are positionedfarther away from the longitudinal centerline 12 of the aircraft 10 thanthe respective PT fans 48, 50, 52, 54. More specifically still, for theembodiment depicted, each of the ST fans 64, 66, 68, 70 are attached toan outer end of the respective wing assemblies 26, 28, along thetransverse direction T.

As with the PT fans 48, 50, 52, 54, each of the plurality of ST fans 64,66, 68, 70 are also electrically connected to the electric power source40 via the primary electric communication bus 42 (and the secondaryelectric communication bus 46 for redundancy purposes). Moreover, forthe embodiment depicted, each of the plurality of PT fans 48, 50, 52, 54and each of the plurality of ST fans 64, 66, 68, 70 are configured aselectric propulsors. Accordingly, each of the plurality of ST fans 64,66, 68, 70 are electrically connected to the electric power source 40and driven by the electric power source 40.

More specifically, referring now also to FIGS. 5 and 6, for theembodiment depicted, each of the plurality of PT fans 48, 50, 52, 54 areconfigured as electric fans, and each of the plurality of ST fans 64,66, 68, 70 are also configured as electric fans. FIG. 5 provides a side,schematic view of a PT fan in accordance with an exemplary embodiment ofthe present disclosure, and FIG. 6 provides a side, schematic view of anST fan in accordance with an exemplary embodiment of the presentdisclosure.

Referring first to FIG. 5, the exemplary PT fan is generally configuredas an unducted electric fan 72. The unducted electric fan 72 generallyincludes a fan section 74 including a plurality of fan blades 76, witheach of the plurality of fan blades 76 extending from a radially outertip 78 to a base 80. Each of the fan blades 76 is attached to a hub 82of the unducted electric fan 72 at the base 80. The hub 82 is attachedthrough a fan shaft 84 to an electric motor 86 position within a cowling88 of the unducted electric fan 72. The electric motor 86 is inelectrical communication with the electric power source 40 via theprimary electric communication bus 42, or more particularly, for theembodiment depicted, through an electric line 90 of the primary electriccommunication bus 42. Notably, the fan section 74 of the unductedelectric fan 72 defines a fan diameter 92, which for the embodimentdepicted refers to a diameter of a circle circumscribing the outer tips78 of the fan blades 76 during operation of the unducted electric fan72.

Referring now to FIG. 6, the exemplary ST fan is generally configured asa ducted electric fan 94. The ducted electric fan 94 similarly includesa fan section 96 including a plurality of fan blades 98, with each ofthe plurality of fan blades 98 extending from a radially outer tip 100to a base 102. Each of the fan blades 98 is attached to a hub 104 of theducted electric fan 94 at the base 102. The hub 104 is attached througha fan shaft 106 to an electric motor 108 positioned within a cowling 110of the ducted electric fan 94. The electric motor 108 is in electricalcommunication with the electric power source 40 via the primary electriccommunication bus 42, or more particularly, for the embodiment depicted,through an electric line 112 of the primary electric communication bus42. The ducted electric fan 94 further includes an outer nacelle 114encircling the fan section 96 and the cowling 110 of the ducted electricfan 94. The cowling 110 and the outer nacelle 114 together define anairflow passage 116. A plurality of struts 118 are provided forconnecting the cowling 110 to the outer nacelle 114. As with theunducted fan section 74, the ducted fan section 96 defines a fandiameter 120.

Although not depicted, one or both of the unducted electric fan 72 orducted electric fan 94 may additionally include a gearbox between arespective electric motor 86, 108 and fan section 74, 96 for increasingor decreasing a rotational speed of the respective fan relative sectionto the respective electric motor 86, 108. Moreover, in certainembodiments, one or both of the unducted electric fan 72 or ductedelectric fan 94 may include one or more mechanisms for varying a pitchof each of the plurality of fan blades 76, 98 during operation.

In certain embodiments, the unducted fan 72 may define the relativelyhigh fan diameter 92 as compared to the ducted fan 94. Additionally, theunducted fan 72 may be configured to generate a relatively high amountsof thrust as compared to the ducted fan 94. Accordingly, the unductedfan 72 may be utilized as a primary source of thrust during takeoffoperating conditions or other vertical flight operations. By contrast,the ducted fans 94 may have a relatively low fan diameter 120 and maygenerate a relatively low amount of thrust. However, the ducted fans 94may operate more efficiently than the unducted fans 72 during certainflight operations. Accordingly, the ducted fans 94 may be utilize as aprimary source of thrust during, e.g., forward flight operations, suchas during cruise operations.

However, as will be appreciated, in other embodiments, one or more ofthe plurality of ST fans 64, 66, 68, 70 may be used in addition to theplurality of PT fans 48, 50, 52, 54 during vertical lift/flightconditions, and additionally or alternatively, one or more of theplurality of PT fans 48, 50, 52, 54 may be used in addition to theplurality of ST fans 64, 66, 68, 70 during horizontal/forward flightconditions. Additionally, as will be discussed with reference to one ormore of the figures below, in certain embodiments, the propulsion system38 may not include each of the plurality of ST fans 64, 66, 68, 70, maynot include each of the plurality of PT fans 48, 50, 52, 54, or mayinclude any other suitable number/form of electric propulsion devices.

Referring still to FIGS. 1 and 2, for the embodiment depicted, theelectric power source 40 is located remotely from the electricpropulsors, within the fuselage 18 of the aircraft 10 proximate the aftend 22 of the fuselage 18. Notably, however, in other embodiments theelectric power source 40 may instead be located at any other suitablelocation within the fuselage 18 of the aircraft 10, or elsewhere.Additionally, the electric power source 40 generally includes acombustion engine and an electric generator 122 driven by the combustionengine for generating electrical power. The combustion engine andelectric generator are mounted, for the embodiment depicted, within thefuselage 18 of the aircraft 10, proximate the aft end 22 of the fuselage18. During operation, the primary electric communication bus 42 connectsthe electric generator 122 to each of the above described electricpropulsors.

Referring now also to FIG. 7, a schematic view of the exemplarycombustion engine and the electric generator 122 is provided. For theembodiment depicted, the combustion engine is configured as a turboshaftengine 124. The turboshaft engine 124 includes in serial flow order, acompressor section including a low pressure compressor 126 and a highpressure compressor 128, a combustion section 130, and a turbine sectionincluding a high pressure turbine 132 and a low pressure turbine 134.During operation, a flow of air is received within the compressorsection and is progressively compressed as it flows therethrough, i.e.,as it flows from the low pressure compressor 126 to the high pressurecompressor 128. The compressed air is then provided to the combustionsection 130 where it is mixed with fuel and burned to generate hotcombustion gas. The hot combustion gas is expanded through the turbinesection where rotational energy is extracted therefrom. Specifically,the hot combustion gas rotates the high pressure turbine 132 and the lowpressure turbine 134 as the gas flows therethrough and is expanded. Asis depicted in phantom, these components may be enclosed within a casing136 within, e.g., the fuselage 18 of the aircraft 10. Although notdepicted, the hot combustion gas may be exhausted, e.g., to atmosphere,from the low pressure turbine 134.

As is also depicted, for the embodiment depicted, the high pressureturbine 132 is connected to the high pressure compressor 128 through ahigh pressure shaft or spool 138, such that a rotation of the highpressure turbine 132 additionally rotates the high pressure compressor128. Similarly, the low pressure turbine 134 is connected to the lowpressure compressor 126 through a low pressure shaft or spool 140, suchthat rotation of the low pressure turbine 134 additionally rotates thelow pressure compressor 126. Moreover, for the embodiment depicted, thelow pressure shaft 140 additionally drives an output shaft 142 extendingto the electric generator 122. Accordingly, a rotation of the turboshaftengine 124 provides rotational energy to the electric generator 122, theelectric generator 122 configured to convert the rotational energy togenerate electrical power. As will be appreciated, in certainembodiments, the electric generator 122 may generally include a rotor144 and a stator 146. The rotational energy of the turboshaft engine 124is provided via the output shaft 142 and configured to rotate the rotor144 of the electric generator 122 relative to the stator 146. Suchrelative movement may generate electrical power.

Inclusion of a turboshaft engine 124 and electric generator 122 inaccordance with such an exemplary embodiment may allow for the electricpower source 40 to generate a relatively high amount of electric powerand to provide such electric power to the plurality of electricpropulsors of the propulsion system 38. For example, in at least certainexemplary embodiments, the turboshaft engine 124 may be a relativelylarge turboshaft engine 124 configured to generate at least about 1000horsepower (“hp”), such that the electric generator 122 generates atleast about 0.75 megawatt (“MW”). Specifically, in certain embodimentsthe turboshaft engine 124 may be configured to generate at least about1320 hp, such that the electric generator 122 generates at least about9.69 MW, such as at least about 1500 hp, such that the electricgenerator 122 generates at least about 1.12 MW, such as at least about1660 hp, such that the electric generator 122 generates at least about1.4 MW. It should be appreciated, that as used herein, terms ofapproximation, such as a “about” or “approximately,” refers to beingwithin 10% margin of error.

In at least certain embodiments, the propulsion system 38 may beconfigured such that the turboshaft engine 124 and electric generator122 are capable of generating a sufficient amount of electrical power todrive each of the electric propulsors of the propulsion system 38simultaneously. By way of example only, for the embodiment where theturboshaft engine 124 generates about 1660 hp, and the generatorgenerates about 1.4 MW, each of the four PT fans 48, 50, 52, 54 mayinclude 175 kW electric motors and similarly, each of the ST fans 64,66, 68, 70 may include 175 kW electric motors. Accordingly, with such anembodiment, the electric propulsion system 38 may be configured tosubstantially fully power each of the electric propulsors during certainoperations, such as during takeoff or other vertical thrust operations.

By contrast, in other embodiments, the propulsion system 38 may beconfigured such that the turboshaft engine 124 and electric generator122 is not capable of simultaneously fully powering each of the electricpropulsors included. By way of example only, for the embodiment wherethe turboshaft engine 124 generates about 1660 hp, and the generatorgenerates about 1.4 MW, each of the four PT fans 48, 50, 52, 54 mayinclude 350 kW electric motors and similarly each of the ST fans 64, 66,68, 70 may include 350 kW electric motors. Accordingly, with such anembodiment, the plurality of PT fans 48, 50, 52, 54 may be configured tooperate during a takeoff or other vertical thrust operation, while theplurality of ST fans 64, 66, 68, 70 may be configured to operate duringforward thrust operations. Additionally, or alternatively, asupplemental power source may be used to drive certain of the propulsorsduring these “peak” operations. For example, in certain embodiments, theturboshaft engine 124 and electric generator 122, in combination withthe one or more energy storage devices 44 of the propulsion system 38,may be used to drive the four PT fans 48, 50, 52, 54, in addition to oneor more of the ST fans 64, 66, 68, 70.

It should be appreciated, however, that the exemplary turboshaft engine124 depicted is provided by way of example only, and that in otherexemplary embodiments, the turboshaft engine 124 may have any othersuitable configuration. For example, in other embodiments, theturboshaft engine 124 may include any other suitable number ofcompressors or turbines, as well as any other suitable number orconfiguration of shafts or spools.

Moreover, it should be appreciated, that in still other embodiments, theelectric power source 40 may also have any other suitable configuration.For example, referring now to FIG. 8, an aircraft 10 and propulsionsystem 38 in accordance with another exemplary embodiment of the presentdisclosure is provided. The exemplary propulsion system 38 depicted inFIG. 8 may be configured in substantially the same manner as exemplarypropulsion system 38 depicted in FIGS. 1 and 2 described above.Accordingly, the same numbers may refer to the same or similar part.

For example, as is depicted, the aircraft 10 generally includes afuselage 18, with a forward wing assembly 26 attached to the fuselage 18proximate a forward end 20 of the fuselage 18 and an aft wing assembly28 attached to the fuselage 18 proximate an aft end 22 of the fuselage18. The forward wing assembly 26 includes a port section with a portforward PT fan 48 and a port forward ST fan 64 attached thereto, and astarboard section with a starboard forward PT fan 50 and a starboardforward ST fan 66 attached thereto. The aft wing assembly 28 similarlyincludes a port section with a port aft PT fan 52 and a port aft ST fan68 attached thereto, and a starboard section with a starboard aft PT fan54 and a starboard aft of the fan attached thereto.

Additionally, the propulsion system 38 includes an electric power source40. The electric power source 40 generally includes a combustion engineand a generator 122. However, for the embodiment depicted, the electricpower source 40 further includes a plurality of combustion engines and arespective plurality of generators 122. Specifically, for the embodimentdepicted, the electric power source 40 includes a first turboshaftengine 124A and a second turboshaft engine 124B. The first turboshaftengine 124A drives a first electric generator 122A and the secondturboshaft engine 124B drives a second electric generator 122B. Thefirst and second turboshaft engines 124A, 124B and first and secondelectric generators 122A, 122B may be configured in substantially thesame manner as exemplary turboshaft engine 124 and electric generator122 described above with reference to FIG. 7. Such a configuration mayallow the electric power source 40 to provide the propulsion system 38with a necessary amount of electric power, and may also provide for aredundancy in the propulsion system 38.

Referring again to FIGS. 1 and 2, for the embodiment depicted, thepropulsion system 38 of the exemplary aircraft 10 depicted is configuredas a substantially balanced propulsion system 38. For example, thepropulsion system 38 includes two forward PT fans (i.e., the port andstarboard forward PT fans 48, 50) and two forward ST fans (i.e., theport and starboard forward ST fans 64, 66), as well as two aft PT fans(i.e., the port and starboard aft PT fans, 52, 54) and two aft ST fans(i.e., the port and starboard aft ST fans 68, 70). The port andstarboard forward PT fans 48, 50 each define a forward PT fan diameter148 and the port and starboard forward ST fans 64, 66 each define aforward ST fan diameter 150. Similarly, the port and starboard aft PTfans 52, 54 each define an aft PT fan diameter 154 and the port andstarboard aft ST fans 68, 70 each define an aft ST fan diameter 156. Forthe embodiment depicted, the forward PT fan diameter 148 issubstantially the same as the aft PT fan diameter 154, and the forwardST fan diameter 150 is substantially the same as the aft ST fan diameter156.

Moreover, for the embodiment depicted, the forward PT fans 48, 50together define a maximum forward PT thrust capability and the aft PTfans 52, 54 also define a maximum aft PT thrust capability. Similarlyfor the embodiment depicted, the forward ST fans 64, 66 together definea maximum forward ST thrust capability and the aft ST fans 68, 70 alsodefine a maximum aft ST thrust capability. For the embodiment depicted,the maximum forward PT thrust capability is substantially the same asthe maximum aft PT thrust capability, and the maximum forward ST thrustcapability is also substantially the same as the maximum aft ST thrustcapability.

Additionally, it will be appreciated that the exemplary aircraft 10depicted defines a minimum necessary takeoff thrust. The minimumnecessary takeoff thrust refers to a minimum amount of vertical thrustrequired for the aircraft 10 to perform a vertical takeoff when carryinga maximum rated weight of cargo. In certain embodiments, the maximumforward PT thrust capability and the maximum aft PT thrust capabilitytogether may be greater than or equal to the minimum necessary takeoffthrust of the aircraft 10. However, in other embodiments, the maximumforward PT thrust capability and the maximum aft PT thrust capabilitytogether may not be greater than or equal to the minimum necessarytakeoff thrust of the aircraft 10. For such exemplary embodiments,however, the maximum forward PT thrust capability and the maximum aft PTthrust capability, together with the maximum forward ST thrustcapability and the maximum aft ST thrust capability, is greater than orequal to the minimum necessary takeoff thrust of the aircraft 10.

It should be appreciated, however, that in other embodiments, theaircraft 10 and propulsion system 38 may instead have any other suitableconfiguration. For example, referring now to FIG. 9, an aircraft 10 andpropulsion system 38 in accordance with another exemplary embodiment ofthe present disclosure is provided. The exemplary propulsion system 38depicted in FIG. 9 may be configured in substantially the same manner asexemplary propulsion system 38 depicted in FIGS. 1 and 2 describedabove. Accordingly, the same numbers may refer to the same or similarpart.

For example, as is depicted, the aircraft 10 generally includes afuselage 18, with a forward wing assembly 26 attached to the fuselage 18proximate a forward end 20 of the fuselage 18 and an aft wing assembly28 attached to the fuselage 18 proximate an aft end 22 of the fuselage18. The forward wing assembly 26 includes a port section with a portforward PT fan 48 and a port forward ST fan 64 attached thereto, and astarboard section with a starboard forward PT fan 50 and a starboardforward ST fan 66 attached thereto. The aft wing assembly 28 similarlyincludes a port section with a port aft PT fan 52 and a port aft ST fan68 attached thereto, and a starboard section with a starboard aft PT fan54 and a starboard aft of the fan attached thereto.

The port and starboard forward PT fans 48, 50 each define a forward PTfan diameter 148. Similarly, the port and starboard aft PT fans 52, 54each define an aft PT fan diameter 154. Additionally, the forward PTfans 48, 50 together define a maximum forward PT thrust capability andthe aft PT fans 52, 54 also define a maximum aft PT thrust capability.However, for the embodiment depicted, the forward PT fan diameter 148 isdifferent than the aft PT fan diameter 154. Specifically, for theembodiment depicted, the aft PT fan diameter 154 is greater than theforward PT fan diameter 148. Moreover, for the embodiment depicted, themaximum forward PT thrust capability is different than the maximum aftPT thrust capability. More specifically, for the embodiment depicted,the maximum aft PT thrust capability is greater than the maximum forwardPT thrust capability.

Notably, the forward ST fans 64, 66 together define a maximum forward STthrust capability and the aft ST fans 68, 70 also define a maximum aftST thrust capability. For the embodiment depicted, the forward ST thrustcapability is substantially the same as the aft ST thrust capability.

It should be appreciated, however, that despite the differences inmaximum thrust capabilities, in at least certain embodiments, themaximum forward PT thrust capability of the forward PT fans 48, 50 andmaximum aft PT thrust capability of the aft PT fans 52, 54 may still begreater than or equal to a minimum necessary takeoff thrust for theaircraft 10. The imbalance of the maximum thrust capabilities betweenthe forward PT fans 48, 50 and aft PT fans 52, 54 may be due at least inpart to a weight distribution of the aircraft 10 or a relativepositioning of the forward wing assembly 26 and aft wing assembly 28.Additionally, or alternatively, the aft PT fans 52, 54 may simply belarger or more powerful to provide for a greater maximum speed of theaircraft 10.

Referring briefly now also to FIG. 10, providing a schematic view of awing section of a wing assembly of an aircraft 10, it should beappreciated that in certain embodiments, one or both of the forward PTfans 48, 50 or aft PT fans 52, 54 are not be required to operate duringcertain flight conditions. For example, during, e.g., cruise operations,the aircraft 10 may receive a desired amount of forward thrust from oneor both of the forward ST fans 64, 66 or aft ST fans 68, 70. With suchan embodiment, at least certain of the PT fans 48, 50, 52, 54 mayinclude a plurality of fan blades 76 that are movable from an extendedposition to a stowed position in order to reduce an amount of drag onthe aircraft 10 from such fan blades 76 when the respective fan is notin use. For example, as is depicted in FIG. 10, the one or more fanblades 76 may fold back adjacent to a core cowling 88 of the fan, suchthat the fan blades 76 create less drag on the aircraft 10. It should beappreciated, however, that in other embodiments, the fan blades 76 maynot fold back when moved from the extended position to the stowedposition, and instead may be configured to at least partially retract,or may be configured to be feathered (i.e., rotated such that a pitchangle of the blades 76 is parallel to an airflow direction).

Notably, in other embodiments the present disclosure, the propulsionsystem 38 may not include all of the propulsors described above. Forexample, referring now to FIG. 11, providing a schematic view of apropulsion system 38 in accordance with another exemplary embodiment ofthe present disclosure, the propulsion system 38 does not include aft STfans 68, 70. In other respects, however, the propulsion system 38 ofFIG. 11 may be configured in substantially the same manner as thepropulsion system 38 described above with reference to FIG. 9. Theexemplary aircraft 10 and propulsion system 38 depicted in FIG. 11 maybe designed with propulsors capable of taking off, while also beingdesigned for efficient cruising operations. For example, a maximum aftPT thrust capability of the aft PT fans 52, 54 may be greater than amaximum forward PT thrust capability of the forward PT fans 48, 50.Moreover, for at least certain exemplary embodiments, a combination ofthe maximum thrust capabilities of the forward PT fans 48, 50 and theaft PT fans 52, 54 may be less than a minimum necessary takeoff thrustfor the aircraft 10. However, for such an exemplary embodiment, theforward ST fans 64, 66 may be configured to assist with takeoffoperations. Accordingly, with such an embodiment, a maximum thrustcapability of the forward ST fans 64, 66, the forward PT fans 48, 50,and aft PT fans 52, 54 may be together be greater than or equal to aminimum necessary takeoff thrust for the aircraft 10.

It should further be appreciated, that the embodiments depicted in FIGS.9 and 11 are also by way of example only. For example, in otherembodiments the forward PT fans 48, 50 may instead define a greatermaximum thrust capability and a greater fan diameter than the maximumthrust capability and fan diameter of the aft PT fans 52, 54. Further,in such an embodiment, the aircraft 10 may or may not include theforward ST fans 64, 66, and instead may include aft ST fans 68, 70.

Moreover, it should be appreciated, that in still other embodiments, thepropulsion system 38 may not include any ST fans mounted to the forwardwing assembly 26 and/or aft wing assembly 28. Additionally, oralternatively, the propulsion system 38 may not include any PT fansmounted to the forward wing assembly 26 and/or the aft wing assembly 28.For example, in certain embodiments, the propulsion system 38 mayinclude PT fans attached to one of the forward wing assembly 26 or aftwing assembly 28, and ST fans attached to the other of the forward wingassembly 26 or the aft wing assembly 28. Additionally, or alternativelystill, the propulsion system 38 may include a greater number of PT fansor ST fans on one or both of the forward wing assembly 26 and/or aftwing assembly 28.

For example, referring now to FIG. 12, a propulsion system 38 of anaircraft 10 in accordance with another exemplary embodiment of thepresent disclosure is depicted. The exemplary propulsion system 38depicted in FIG. 12 may be configured in substantially the same manneras exemplary propulsion system 38 described above with reference toFIGS. 1 and 2. Accordingly, the same numbers may refer to the same orsimilar parts.

As is depicted, the aircraft 10 generally includes a fuselage 18extending between a forward end 20 and aft end 22, with a forward wingassembly 26 attached to the fuselage 18 proximate the forward end 20 andan aft wing assembly 28 attached the fuselage 18 proximate the aft end22. The propulsion system 38 additionally includes a port forward PT fan48, a starboard forward PT fan 50, a port aft PT fan 52, and a starboardaft PT fan 54. However, for the embodiment depicted, each of the PT fansare instead configured as a plurality of PT fans. More specifically, forthe embodiment depicted, the port forward PT fan 48 includes a pluralityof PT fans, the starboard forward PT fan 50 includes a plurality of PTfans, the port aft PT fan 52 includes a plurality of PT fans, and thestarboard aft PT fan 54 includes a plurality of PT fans. Morespecifically, still, for the embodiment of FIG. 12, the port forward PTfan 48 includes a pair of PT fans, the starboard forward PT fan 50includes a pair of PT fans, the port aft PT fan 52 includes a pair of PTfans, and the starboard aft PT fan 54 includes a pair of PT fans.

Although the propulsion system 38 of FIG. 12 includes two fans mountedto each of the port and starboard sides of the forward and aft wingassemblies 26, 28, in other embodiments, the propulsion system 38 mayinstead include any other suitable number of fans mounted to the wingassemblies. For example, in certain embodiments, the propulsion system38 may include a plurality of PT fans and/or ST fans mounted to each ofthe port and starboard sides 30, 32, 34, 36 of one of the forward wingassembly 26 or aft wing assembly 28, and a different number of PT fansand/or ST fans mounted to the port and starboard sides 30, 32, 34, 36 ofthe other of the forward wing assembly 26 or aft wing assembly 28.

Further still, in other embodiments, the propulsion system 38 may haveany other suitable type of propulsors. For example, referring now toFIG. 13, a propulsion system 38 for an aircraft 10 in accordance withyet another exemplary embodiment of the present disclosure is depicted.The exemplary propulsion system 38 and aircraft 10 of FIG. 13 may beconfigured in substantially the same manner as exemplary propulsionsystem 38 described above with reference to FIGS. 1 and 2. Accordingly,the same numbers may refer to the same or similar part.

For example, the aircraft 10 generally includes a fuselage 18 extendingbetween a forward end 20 and an aft end 22 generally along alongitudinal centerline 12. A forward wing assembly 26 is attached tothe fuselage 18 proximate the forward end 20 of the fuselage 18 and anaft wing assembly 28 is attached to the fuselage 18 proximate the aftend 22 of the fuselage 18. The exemplary propulsion system 38 includes aplurality of propulsors attached to one or both of the forward wingassembly 26 and the aft wing assembly 28. More particularly, for theembodiment depicted, the propulsion system 38 includes a plurality ofport side propulsors (e.g., a port forward PT fan 48, a port aft PT fan52, a port forward ST fan 64, and a port aft ST fan 68) and a pluralityof starboard side propulsors (e.g., a starboard forward PT fan 50, astarboard aft PT fan 54, a starboard forward ST fan 66, and a starboardaft ST fan 70). Each of these propulsors is in electrical communicationwith a remotely positioned electric power source 40 via a primaryelectric communication bus 42 and a secondary electric communication bus46.

Moreover, for the embodiment depicted, the exemplary propulsion system38 further includes a supplemental propulsor mounted to the fuselage 18of the aircraft 10. For the embodiment depicted, the supplementalpropulsor is configured as an aft fan 158, and more particularly, aducted aft fan attached to the fuselage 18 at the aft end 22 of thefuselage 18. However, by contrast with the other propulsors of thepropulsion system 38, the aft fan 158 is mechanically coupled to thecombustion engine (i.e., the turboshaft engine 124) via a supplementalfan shaft 160. As is depicted in phantom, the propulsion system 38 mayinclude a gearbox 162 through which the aft fan 158 is mechanicallycoupled to the turboshaft engine 124. The gearbox 162 may be a reductiongearbox for reducing rotational speed of the aft fan 158 relative to theturboshaft engine 124, or alternatively, the gearbox 162 may increase arotational speed of the aft fan 158 relative to the turboshaft engine124. As is also depicted in phantom, the propulsion system 38 mayadditionally include a coupling unit 164, such that the aft fan 158 isselectively mechanically coupled to the turboshaft engine 124 throughthe coupling unit 164. The coupling unit 164 may include, e.g., a clutchor other similar coupling means. Moreover, in certain embodiments, thepropulsion system 38 may additionally include one or more gears, suchoffset gears, linkages, etc. (not shown) for mechanically coupling theaft fan 158 to the turboshaft engine 124 via the aft fan 158 shaft 160.

Referring now also to FIG. 14, a side, cross-sectional view is providedof the aft fan 158 of the exemplary propulsion system 38 of FIG. 13. Theaft fan 158 defines a centerline axis 166 that, for the embodimentdepicted, aligns with the longitudinal centerline 12 of the aircraft 10,and further aligns with a mean line 24 of the aircraft 10. As shown, theexemplary aft fan 158 generally includes a plurality of fan blades 168rotatable about the centerline axis 166 by the fan shaft 160.Specifically, each of the plurality of fan blades 168 are attached at abase 170 to a hub 172, the hub 172 coupled with the shaft 160.

The aft fan 158 additionally includes a plurality of forward supportmembers 174, or struts, an outer nacelle 176, and a tail cone 178. Theplurality of forward support members 174 are spaced along acircumferential direction C (i.e., a direction extending about thecenterline axis 166; not shown) and extend between the fuselage 18 andthe outer nacelle 176. The outer nacelle 176 extends substantially threehundred and sixty degrees (360°) around the mean line 24 of the aircraft10, and the centerline axis 166 of the aft fan 158. Accordingly, the aftfan 158 also defines an inlet 180 at a forward end that also extendssubstantially 360° around the mean line 24 of the aircraft 10 and aroundthe centerline axis 166 of the aft fan 158. Notably, the forward supportmembers 174 may act as inlet guide vanes for the aft fan 158.Additionally, in certain embodiments, the aft fan 158 may additionally,or alternatively, include aft support members positioned aft of theplurality of fan blades 76 extending between the outer nacelle 176 andthe tail cone 178.

During operation of the aft fan 158, the aft fan 158 is configured toingest a flow of boundary layer air flowing over an outer surface offuselage 18. The aft fan 158 receives the boundary layer air through theinlet 180 and re-energizes such flow of air through rotation of theplurality of fan blades 168. Notably, as discussed above, the pluralityof fan blades 168 are rotatable by the turboshaft engine 124 of thepropulsion system 38 through the fan shaft 160. The re-energized flow ofair exits through a nozzle 182 defined between the outer nacelle 176 andthe tail cone 178. The re-energized air may generate thrust through thenozzle 182, or alternatively, the re-energized air may simply reduce anamount of drag on the aircraft 10.

Moreover, referring still to FIG. 14, the aft fan 158 of the exemplarypropulsion system 38 depicted may further include a thrust augmenter 184movable between a forward thrust position and a vertical thrust position(depicted in phantom in the vertical thrust position). The thrustaugmenter 184 may, when in the vertical thrust position, be configuredto redirect the airflow through the nozzle 182 of the aft fan 158 suchthat the aft fan 158 generates substantially vertical thrust. For theembodiment depicted, the thrust augmenter 184 includes a plurality ofnacelle extensions 186, which may be embedded within the outer nacelle176 when in a forward thrust position, for extending and pivoting an aftportion of the outer nacelle 176. Additionally, the thrust augmenter 184includes a plurality of tail cone extensions 188, for extending andpivoting the tail cone 178. However, in other embodiments, the thrustaugmenter 184 may be configured in any other suitable manner.

Referring now to FIG. 15, the exemplary method (200) of operating apropulsion system of an aircraft in accordance with an exemplary aspectof the present disclosure is provided. The method (200) of FIG. 15 maybe utilized with one or more of the exemplary propulsion systems andaircraft described above with reference to FIGS. 1 through 14.Accordingly, the propulsion system may include a plurality of forwardpropulsors and a plurality of aft propulsors, each powered by anelectric power source and rotatable between a forward thrust positionand a vertical thrust position.

As is depicted, the exemplary method (200) includes at (202) moving theplurality of forward propulsors and the plurality of aft propulsors tothe vertical thrust positions. Additionally, the exemplary method (200)includes at (204) providing a first forward to aft ratio of electricpower to the plurality of forward propulsors and the plurality of aftpropulsors from the electric power source, such that the plurality offorward propulsors and the plurality of aft propulsors each generatevertical thrust. Providing the first forward to aft ratio of electricpower at (204) may include providing the first forward to aft ratio ofelectric power during one or more of a takeoff operating mode, ahovering operating mode, or a landing operating mode.

Additionally, the exemplary method (200) includes at (206) moving theplurality of forward propulsors and the plurality of aft propulsors tothe forward thrust positions. Once in the forward thrust positions, theexemplary method (200) includes at (208) providing a second forward toaft ratio of electric power to the plurality of forward propulsors andthe plurality of aft propulsors from the electric power source, suchthat one or more of the plurality of forward propulsors and theplurality of aft propulsors generate a forward thrust.

Notably, for the exemplary method (200), the first forward to aft ratioof electric power is different than the second forward to aft ratio ofelectric power. For example, in certain aspects, the first forward toaft ratio of electric power is greater than the second forward to aftratio of electric power. Alternatively, however, in other aspects, thesecond forward to aft ratio of electric power is greater than the firstforward to aft ratio of electric power. Such an exemplary aspect allowfor the propulsion system to include certain propulsors configured foruse during, e.g., vertical thrust operations, such as takeoff, hovering,and landing operations, while other propulsors are additionallyconfigured for use during, e.g., cruise operations.

As will be appreciated, from the above description, in certain aspects,the plurality of forward propulsors may be configured as a plurality offorward primary propulsors and the propulsion system may further includea plurality of forward supplemental propulsors. Similarly, the pluralityof aft propulsors may be configured as a plurality of aft primarypropulsors and the propulsion system may further include a plurality ofaft supplemental propulsors. With such an exemplary embodiment, theexemplary method (200) may additionally include at (210) providing afirst amount of electrical power from the electric power source to theplurality of forward and aft supplemental propulsors when the pluralityof forward primary propulsors and the plurality of aft primarypropulsors are in the vertical thrust positions. The method mayadditionally include at (212) providing a second amount of electricalpower from the electric power source to the plurality of forward and aftsupplemental propulsors when the plurality of forward primary propulsorsand aft primary propulsors are in the forward thrust positions. For theexemplary aspect depicted, the second amount of electrical power isgreater than the first amount of electrical power. For example, incertain exemplary aspects, the first amount of electrical power may beless than about half of the second amount of electrical power. With suchan exemplary aspect, the primary propulsors may be most suitable forvertical thrust operations, and the supplemental propulsors may be mostsuitable for forward thrust operations. Specifically, with such anexemplary aspect, the primary propulsors may be used for, e.g., takeoff,hovering, and landing operations, while the supplemental propulsors maybe used for, e.g., cruise operations.

Referring still to the exemplary aspect of FIG. 15, the exemplary method(200) may further apply to a propulsion system for an aircraft furtherincluding a supplemental propulsor mounted to a fuselage of theaircraft. The supplemental propulsor may be mounted at an aft end of thefuselage, and may further be configured to ingest and re-energize aboundary layer air flowing over the fuselage of the aircraft.Accordingly, in certain exemplary aspects, the supplemental propulsormay be a boundary layer ingestion aft fan. With such an exemplaryaspect, the exemplary method (200) further includes at (214) providing afirst amount of power from the electric power source to the supplementalpropulsor when the plurality of forward propulsors and the plurality ofaft propulsors are in the vertical thrust positions. The exemplarymethod (200) further includes at (216) providing a second amount ofpower from the electric power source to the supplemental propulsor whenthe plurality of forward propulsors and the plurality of aft propulsorsare in the forward thrust position. For the exemplary aspect depicted,the second amount of power is greater than the first amount of power.For example, the first amount of power may be less than about half ofthe second amount of power. Accordingly, for the aspect depicted, thesupplemental propulsor may be generally be configured for use duringforward flight operations, such as during cruise operations.

Additionally or alternatively, however, the propulsion system mayfurther include a thrust augmenter configured with the supplementalpropulsor. With such an embodiment, the method (200) may include movingthe thrust augmenter to a vertical thrust position when the primarypropulsors are in the vertical thrust positions, and moving the thrustaugmenter to a forward thrust position when the primary propulsors arein the forward thrust positions. Alternatively still, the method (200)may instead include moving the supplemental propulsor between a verticalthrust position and a forward thrust position.

Referring now to FIG. 16, providing a flow diagram of a method (300) inaccordance with another exemplary aspect of the present disclosure, themethod (300) may be utilized with a propulsion system in accordance withone or more of the above embodiments including a plurality of primarythrust propulsors and a plurality of secondary thrust propulsors.

The method (300) includes at (302) moving the plurality of primarythrust propulsors to the vertical thrust positions. In certain exemplaryaspects, moving the plurality of primary thrust propulsors at (302) tothe vertical thrust positions may further include moving the pluralityof secondary thrust propulsors to the vertical thrust positions. Themethod (300) further includes at (304) providing power to the pluralityof primary thrust propulsors to generate a vertical thrust forvertically oriented flight and simultaneously providing a first amountof electrical power to the plurality of secondary thrust propulsors.

The exemplary method (300) further includes at (306) moving theplurality of primary thrust propulsors to the forward thrust positions.Again, in certain exemplary aspects, moving the plurality of primarythrust propulsors at (306) to the forward thrust positions may includemoving the plurality of secondary thrust propulsors to the forwardthrust positions. The method (300) next includes at (308) providing asecond amount of electrical power to the plurality of secondary thrustpropulsors to generate a forward thrust for horizontally orientedflight. For the exemplary aspect depicted, the second amount ofelectrical power is greater than the first amount of electrical power,such that the plurality secondary thrust propulsors generate more thrustin the forward flight positions and during forward flight than when inthe vertical thrust positions and during vertically oriented flight.

In certain exemplary aspects, the second amount of electrical power isat least about two times greater than the first amount of electricalpower, such as at least about four times greater than the first amountof electrical power. Moreover, in certain exemplary aspects, the firstamount of electrical power may be less than 10% of the second amount ofelectrical power. Moreover, the aircraft with which the propulsionsystem is configured may define a minimum necessary takeoff thrust. Withsuch an exemplary aspect, providing power to the plurality primarythrust propulsors to generate vertical thrust at (304) may includeproviding power to the plurality of primary thrust propulsors togenerate a vertical thrust within at least about 10% of the minimumnecessary takeoff thrust.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. An aircraft comprising: a fuselage extending between a forward end and an aft end; a forward wing assembly attached to or formed integrally with the fuselage proximate the forward end of the fuselage; an aft wing assembly attached to or formed integrally with the fuselage proximate the aft end of the fuselage; and a propulsion system comprising a port forward propulsor and a starboard forward propulsor, the port and starboard forward propulsors each attached to the forward wing assembly on opposing sides of the fuselage and rotatable between a forward thrust position and a vertical thrust position, the port and starboard forward propulsors together defining a maximum forward thrust capability; and a port aft propulsor and a starboard aft propulsor, the port and starboard aft propulsors each attached to the aft wing assembly on opposing sides of the fuselage and rotatable between a forward thrust position and a vertical thrust position, the port and starboard aft propulsors together defining a maximum aft thrust capability, the maximum forward thrust capability being different than the maximum aft thrust capability.
 2. The aircraft of claim 1, wherein the maximum aft thrust capability is greater than the maximum forward thrust capability.
 3. The aircraft of claim 2, wherein the port and starboard forward propulsors each include a fan having a plurality of fan blades, wherein the plurality of fan blades are moveable to a stowed position when the port and starboard forward propulsors are rotated to the forward thrust position.
 4. The aircraft of claim 1, wherein the maximum forward thrust capability is greater than the maximum aft thrust capability.
 5. The aircraft of claim 4, wherein the port and starboard aft propulsors each include a fan having a plurality of fan blades, wherein the plurality of fan blades are moveable to a stowed position when the port and starboard aft propulsors are rotated to the forward thrust position.
 6. The aircraft of claim 1, wherein the port and starboard forward propulsors each define a fan diameter, wherein the port and starboard aft propulsors each define a fan diameter, and wherein the fan diameter of the port and starboard aft propulsors is different than the fan diameter of the port and starboard forward propulsors.
 7. The aircraft of claim 1, wherein the aircraft defines a minimum necessary takeoff thrust, and wherein the maximum forward thrust capability in combination with the maximum aft thrust capability is greater than or equal to the minimum necessary takeoff thrust of the aircraft.
 8. The aircraft of claim 1, wherein the port and starboard forward propulsors are configured as electric fans, and wherein the port and starboard aft propulsors are also configured as electric fans.
 9. The aircraft of claim 8, wherein the propulsion system further comprises an electric power source located remotely from the port and starboard forward propulsors and from the port and starboard aft propulsors, wherein the electric power source is in electric communication with the port and starboard forward propulsors and the port and starboard aft propulsors.
 10. The aircraft of claim 9, wherein the electric power source powers the port and starboard forward propulsors and the port and starboard aft propulsors.
 11. The aircraft of claim 9, wherein the electric power source comprises a combustion engine and an electric generator, wherein the combustion engine powers the electric generator.
 12. A propulsion system for an aircraft, the aircraft including a forward wing assembly and an aft wing assembly, the forward and aft wing assemblies each attached to or formed integrally with the fuselage, the propulsion system comprising: a port forward propulsor and a starboard forward propulsor, the port and starboard forward propulsors each configured for attachment to the forward wing assembly on opposing sides of the fuselage and rotatable between a forward thrust position and a vertical thrust position, the port and starboard forward propulsors together defining a maximum forward thrust capability; and a port aft propulsor and a starboard aft propulsor, the port and starboard aft propulsors each configured for attachment to the aft wing assembly on opposing sides of the fuselage and rotatable between a forward thrust position and a vertical thrust position, the port and starboard aft propulsors together defining a maximum aft thrust capability, the maximum forward thrust capability being different than the maximum aft thrust capability.
 13. The propulsion system of claim 12, wherein the maximum aft thrust capability is greater than the maximum forward thrust capability.
 14. The propulsion system of claim 13, wherein the port and starboard forward propulsors each include a fan having a plurality of fan blades, wherein the plurality of fan blades are moveable to a stowed position when the port and starboard forward propulsors are rotated to the forward thrust position.
 15. The propulsion system of claim 12, wherein the maximum forward thrust capability is greater than the maximum aft thrust capability.
 16. The propulsion system of claim 15, wherein the port and starboard aft propulsors each include a fan having a plurality of fan blades, wherein the plurality of fan blades are moveable to a stowed position when the port and starboard aft propulsors are rotated to the forward thrust position.
 17. The propulsion system of claim 12, wherein the port and starboard forward propulsors each define a fan diameter, wherein the port and starboard aft propulsors each define a fan diameter, and wherein the fan diameter of the port and starboard aft propulsors is different than the fan diameter of the port and starboard forward propulsors.
 18. The propulsion system of claim 12, wherein the aircraft defines a minimum necessary takeoff thrust, and wherein the maximum forward thrust capability in combination with the maximum aft thrust capability is greater than or equal to the minimum necessary takeoff thrust of the aircraft.
 19. The propulsion system of claim 12, wherein the port and starboard forward propulsors are configured as electric fans, and wherein the port and starboard aft propulsors are also configured as electric fans.
 20. The propulsion system of claim 19, wherein the propulsion system further comprises an electric power source located remotely from the port and starboard forward propulsors and from the port and starboard aft propulsors, wherein the electric power source powers the port and starboard forward propulsors and the port and starboard aft propulsors. 